Method of determining strain condition of the eye



F. ZANDMAN Oct. 22, 1968 METHOD OF DETERMINING STRAIN CONDITION OF THEEYE 7 Sheets-Sheet 1 Filed May 15, 1963 FIG.

INVENTOR.

FELIX Z4/VOMfl/V F. ZANDMAN Oct. 22, 1968 METHOD OF DETERMINING STRAINCONDITION OF THE EYE 7 Sheets-Sheet 2 Filed May 13, 1965 FIG /7INVENTOR. F E L IX 2 H/VDMAN F. ZANDMAN 3,406,681

METHOD OF DETERMINING STRAIN CONDITION OF THE EYE Oct. 22, 1968 7Sheets-Sheet 5 Filed May 15, 1963 FIG. /9

BY v 4%! A'TTOR/VEY Oct. 22, 1968 ZANDMAN 3,406,681

METHOD OF DETERMINING STRAIN CONDITION OF THE EYE Filed May 13, 1963 '7Sheets-Sheet 4 FIG. 26 F/G; 27

7' V CAMERA 7' V Rf (E/V5 INVENTOR.

F EL /X ZAWOMfl/V i9 PUMP BY 7' TORNE Y Oct. 22, 1968 F. ZANDMAN3,406,681

METHOD OF DETERMINING STRAIN CONDITION OF THE EYE Filed May 13, 1963 7Sheets-Sheet 5 FIG. 33

INVENTOR.

FEL IX ZIIVDMfl/V Oct. 22, 19 68 F. ZANDMAN 3,406,681

METHOD OF DETERMINING STRAIN CONDITION OF THE EYE Filed May 13, 1963 7Sheets-Sheet 6 NORMAL [YE OBSERVED ABNORMAL Y OBSt'Rl/'D 77/2006 APOLAR/JCOPf THROUGH A POLAR/SCOPE FIG: 36

Oct. 22, 1968 F. ZANDMAN 3,406,681

METHOD OF DETERMINING STRAIN CONDITION OF THE EYE Filed May 13, 1963 '7Sheets- Sheet '7 COMPENSATOR QUARTER WAVE PLATE ANALYZER POLARIZERCORNEA OBSERVER LIGHT SOURCE P,QLU=POLARIZER AND QUARTER WAVE PLATE HALFMIRROR A,Q EANALYZER AND QUARTER WAVE PLATE fix. NOSE MIRROR PUPIL OFEYE SLAVE SYSTEM (SERVOMOTOR) P,Q,S,O-W|TH NUMERAL SUBSCRIPTS MEANCONSECUTIVE OR SEPARATE POLARIZERS, QUARTER WAVE PLATES, LIGHT SOURCESAND OBSERVERS AND THE SUBSCRIPT Q MEANS THAT THERE MAY BE ANY NUMBERTHEREOF.

NUMERALS O,I,2, 2.7, AND 3 INDICATE IN NORMAL AND ABNORMAL EYESINVENTOR.

FELIX ZANDMAN TTORNEY FRINGE ORDERS BY United States Patent 3,406,681METHOD OF DETERMINING STRAIN CONDITION OF THE EYE Felix Zandman,Villanova, Pa., assignor to Vishay Intertechnology, Inc., Malvern, Pa.,a corporation of Pennsylvania Filed May 13, 1963, Ser. No. 279,978 9Claims. (Cl. 128--2) This invention relates to the study, determinationand measurement of the strain condition of human and animal tissue andmore particularly to a method and means for determining and measuringthe intraocular pressure of the living eye and hence for diagnosingabnormal eye conditions especially in the case of glaucoma. Theinvention makes use of polarized light, and takes advantage of thephenomenon of organized birefringence in the eye, with optical axisdirections constant through out the cornea thickness.

Certain transparent materials when subjected to stress or strain exhibitthe phenomenon of double refraction or birefringence. They divide anincident beam of light into two beams which travel through such materialat different speeds. This phenomenon occurs whether or not the incidentlight is polarized. This means that one beam will be retarded in respectto the other while traveling through the material. This relativeretardation a, of one beam in respect to the other divided by thethickness t of the material through which the light traveled is calledbirefringence. In addition, the two beams traveling through thisstressed material are polarized at right angles to each other. Theirdirections of polarization are parallel and perpendicular to the twodirections of principal stresses in the material. Those two beams oflight will interfere and two sets of fringes or bands (black bands andone color band) will be visible if monochromatic polarized light is usedto illuminate the material and an analyzer comprising a furtherpolarization-selective element is used for observation of the material.One set of fringes called isochromatics is related to the stressmagnitudes in the material; the other set of fringes called isoclinicsis related to the directions of principal stresses in the material. Thebirefringence results directly from and is directly related to thestrain in the material through which the light passes, and is thereforerelated also to the stress which produces that strain. When whitepolarized light is used for illumination, isochromatic fringes appear'as color bands and isoclinic fringes as black bands. When directions ofprincipal strains (or stresses) vary through the material thickness,isoclinics are not visible. Newmann has shown that relative retardationwherein in and n are the two principal indexes of refraction in a planeperpendicular to the beam of light, t is thickness of material traversedby light, 0 and 0' are the two principal stresses in a planeperpendicular to the beam of light, and C is the stress opticalconstant. The same formula can be written in terms of strain as follows:

Here 6 and 5 are the principal strains and k is the strain opticalconstant. 0' 0' 6 and 6 are secondary principal stresses and strains ifthe beam of light is perpendicular to a non-principal plane. If thelight is traversing the material twice, for example, if a mirror isprovided in the back of the transparent material, hence bouncing backthe light, the light traverses the material a second time, and then 1would be qual to twice the thickness of the material. 6 can bedetermined by sev- 3,406,681 Patented Oct. 22, 1968 eral methods such ascolor chart comparison, fringe counting, compensation or photometricmethod. 6 measured under normal light incidence conditions provides thedifference of principal stresses or strains. Measurement of 6 underoblique light incidence conditions provides additional data so thatseparate values of principal stresses or strains are obtained. Thusstrains resulting from the stresses can be quantitatively determined inbirefringent materials such as the cornea of an eye or otherbirefringent tissues. The technique of stress measurement inbirefringent materials is called Photoelasticity. For more details onthis subject, reference is made to the Non-Destructive Testing Handbook,1959,

edited by McMaster, Ronald Press, pages 53-1 to 53-39, volume 2, chapterentitled Photoelastic Coating Tests. Other literature on photoelasticitywhere the same phenomenon is described will be found in the bibliographycited at the end of the article. Additional information onphotoelasticity is available in ASME Handbook, MetalsEngineering-Design, 2nd ed., 1965, McGraw-Hill Book Co., pp. 554-582,and in the two bibliographies included therein.

It is known that the eye is an extremely delicate organ which makes itdifficult in some instance to examine it medically to determine itscondition having in mind making a diagnosis or following the progress ofa disease. Direct observation with or without magnification providesinformation only in certain special circumstances and is only of limiteduse in connection with various diseases notably those of interest in thedepth or mass of the eye. This is the case, for example, in glaucoma.For the beneficial treatment of this malady and the study of itsprogress there has only been available up to now an apparatus known as atonometer of very delicate manipulation reserved only to specialists forthe purpose of making a direct measurement of the compressibility of theeyeball by the application of a known force.

The present invention is predicated on the discovery that the eye corneapossesses the property of organized birefringence or in other words:that the principal strain directions are constant through the corneathickness as evidenced by the presence of sharp well-defined isoclinics,and that the location of the isoclinic and isochromatic fringes can beobserved, recorded and measured for diagnostic purposes or for followingthe course of a disease which causes changes in the intraocular pressureof the eye as in glaucoma. The invention, however, is useful fordetermining whether the eye is normal or undiseased and by making aseries of observations and measurements over a period of time favorableor unfavorable progress of the condition of the eye can be ascertained.The invention makes it possible both on a qualitative and on aquantitative basis to observe, record and measure the birefringence ofthe eye and without making any physical contact with the eye since theinvention is primarily an optical method and apparatus. The inventionthus adds a very important research and/ or diagnostic tool to thetechniques heretofore available to ophthalmologists and other physiciansor technicians specializing in diseases of the eye.

It has now been found that if one observes a human eye or pair of eyeswith apparatus designed for the observation and study of birefringence,it can be verified that the phenomenon of .birefringence does exist inthe eye and can be observed, measured and photographed to obtaininformation concerning the condition of the eye and to revealabnormalities rapidly and accurately and to disclose differences betweenthe eyes of the same person.

The invention thus permits the practitioner or ophthalmologist toarrange means entirely novel for him which make it possible to carry outa diagnosis, to followthe progress of a disease, to judge the effect oftreatment and I all without harm to the patient which contributes adevelopment of important value to the care and therapy of the eye.

According-to the invention, observations and measurements are carriedout on the eye analogous to those which have been believed to berestricted heretofore to devices or components in the field of mechanicsand hydraulics, notably to those subjected to natural or artificialconditions. Y

The invention envisions the application in the study of the eye ofnumerous methods used in the above fields and equally means andcomponents of apparatus utilized in said fields thus enriching to animportant extent the armamentarium at the disposal of theophthalmologist.

In the-following description reference is made to the annexed drawings,in which:

FIGURE 1 is a fragmentary sectional schematic view of a portion of ahuman eye;

FIGURE 2 is a diagrammatic view of the preferred embodiment of apparatusapplying the procedure of the present invention;

FIGURE 3 is a front view of the apparatus of FIG- URE 2;

FIGURE 4 is a view similar to FIGURE 2 but for a modification into afigure of revolution;

FIGURE 5 is a face view corresponding to the embodiment of FIGURE 4;

FIGURES 6 and 7 illustrate schematically the isochromatic lines observedin the normal right and left eyes when operating in accordance with thepresent invention in a crossed circular and parallel circularpolariscope;

FIGURES 8 and 9 are the relative retardation graphs corresponding to theeye conditions shown in FIGURES 6 and 7;

FIGURE 10 is a schematic view of an isoclinic pattern observed undercrossed linear polariscope conditions;

FIGURE 11 is a schematic view of another arrangement of apparatus;

FIGURES 12 and 13 are detail perspective views of a component part ofsuch arrangement of apparatus;

FIGURE 14 is a schematic view of another arrangement of apparatus forcarrying out the invention with the ray emerging from the examined eyecoincident with the incident ray;

FIGURE 15 is a view similar to FIGURE 14 but for a further modificationfor self-observation;

FIGURE 16 is a view similar to the two preceding views but for a stillfurther modification;

FIGURE 17 is a view similar to the preceding figures but for stillanother modification;

FIGURE 18 shows at K a corrective lens interposed in front of the eye tobe observed;

FIGURE 19 shows schematically an apparatus arrangement for an additionalembodiment;

FIGURE 20 is also a schematic illustration for still another embodiment;

FIGURE 21 is a schematic showing of a still further embodiment;

FIGURE 22 shows schematically an isochromatic pattern observed in theeye when using the present invention and wherein the broken linescorrespond to oblique incidence observation;

FIGURE 23 schematically illustrates another form of apparatus;

FIGURE 24 schematically illustrates a different mounting arrangement;

FIGURE 25 schematically shows a further mounting arrangement;

FIGURE 26 shows schematically still another mounting arrangement;

FIGURE 27 schematically illustrates an apparatus according to theinvention;

FIGURE 28 is similar to FIGURE 27 but for a modified arrangement;

FIGURE 29 is similar to FIGURES 27 and 28 but for still anotherembodiment;

FIGURE 30 schematically illustrates an arrangement of apparatus providedfor use by more than one observer;

FIGURE 31 is a view similar to FIGURE 30 but for difierent conditions ofobservation;

FIGURE 32 is a schematic view of a calibration measuring arrangement; IV 'f FIGURE 33 is a front elevational 'schematicyiew of the isochromaticfringe pattern of a normal eye; A

FIGURE34 is similar to FIGURE33 but for abnormal left and right eyes;

FIGURE 35 is similar to FIGURE 34 but showing fringe patterns of otherabnormal eyes;

FIGURE 36 is similar to FIGURES 34 and 35 wherein still otherabnormalities exist;

FIGURE 37 is similar to FIGURES 34 through 36 but wherein one eye isnormal and the other eyeis abnormal;

FIGURE 38 is a diagrammatic"representation -of a scale for "measuringthe position of the fringe for a normal eye and showing the fringeportion of the examined eye on'the scale; j

FIGURE 39 is a diagrammatic representation of a composite scalearrangement which is adapted to make both circular and linearmeasurements and showing the fringe portion of the examined eye thereon;and

FIG. 40 is a table of explanatory legends.

In FIGURE 1 there has been fragmentarily shown in transverse section theanterior portion of the eye comprising the cornea 1 and the iris 2bordering pupil'3 behind which is the crystalline lens 4. This structureis shown at e in FIGURE. 2..which is a diagram of an installation forthe observation of the birefringence of the eye. The apparatus isprovided for'the observation of the normal or quasi-normal angle ofincidence. Light from source S projects through a polarizer P andeventually through a quarter-wave plate Q which has its optical axisturned 45 from the polarizer axis, the light proceeding thence into thepatients eye e. This light passes through the cornea, impinges on and isreflected by the surface of the iris, and the resultant outwardlydirected ray of light passes again through the cornea and thence throughcompensator C, quarter-wave plate Q, and analyzer A, at which point thelight may be viewed by an observer or photographed. In this apparatusas-well as in the forms of apparatus to be described hereinafter thelight source S can be a source of monochromatic or polychromatic light,that is to say, a white light, or even a partially filtered light. Itcan be generated by an incandescent lamp, a stroboscope or a clear lamp,or an electronic flash or a source of modulated light. Apparatus whichmay be used in accordance with FIG. 2 is shown and described in US.Patent No. 3,062,087, issued Nov. 6, 1962, to Felix Zandman and JeanAvril.

FIGURE 3 shows in face view a polariscope comprising on its left portionan analyzer A and a quarter-wave plate Q and on its right side apolarizer P and a quarterwave plate Q. The quarter-wave plates can bemounted in such manner that they can be either in an operative or in aninoperative position. The apparatus comprises means known in itself toput it in rotation according to therelated movements of the polarizerand the analyzer, an embodiment being that the quarter-wave platesrespectively participate correspondingly with the polarizer and the.analyzer.

In FIGURES 4 and 5, the arrangement of FIGURES 2 and 3 has been modifiedinto a figure of revolution about the axis of the eye, with the sourceS, polarizer P and quarter-wave plate Q surrounding the analyzer A whichis between the compensator and the observers eye, whereby observation ismade between the polarizer P and light source S through compensator Cand quarter-wave plate and analyzer Q and A, respectively.

FIGURES 6 and 7 schematically show the appearance presented by thecentral portion of the left and right eyes to the observer through apolariscope in crossed circular and parallel circular conditions. Inthese two figures the line is the outline or periphery of the iris,i,e., the pigmented portion of the eye, which is relied upon to reflectthe incident polarized light back through the cornea toward theobserver. The light directed through the polarizer toward the patientseye is twice refracted by the cornea, first in passing through to theiris, and then, after reflection, in passing outward through the cornea.The images presented to the observer by the patients right and lefteyes, as seen in FIGS. 6 and 7, respectively, include a central area 3resulting because the rays to the center of the iris pass through theaperture and are not reflected back by the iris. These images also eachinclude a pattern which is generally of the shape of a rounded squarehaving its sides disposed diagonally. Each such image includes two dots9 and one to the left of spot 3 and one to its right. FIGS. 6 and 7 eachshow a superposition of three such rounded-square fringe patterns, anyone of which is obtainable by angular adjustment of the analyzer A shownin FIG. 2.

FIG. 8 shows a plot of the distances above and below center of thepattern in FIG. 6 of the points of intersection of the Y axis with therounded square isochromatic pattern as these distances vary with thechange of angle of the analyzer. FIG. 9 shows a plot, of the distancesto right and left of center, of the points of intersection of the axiswith the isochromatic pattern as a function of angle of the analyzer.

FIGURE 10 shows on a larger scale the isoclinic fringes observable withthe aid of a linear crossed polariscope with a normal angle of incidencefor the respective isoclinic parameters of 30, 45, 60, 75, 0 and 90, 0being a direction parallel to the median line of the patients face n.

Only the portion of the isoclinic fringe patterns to the right are shownin FIG. 10 in solid line. The corresponding portions to the left aresituated equal distances in the opposite direction from the center ofthe pattern, as shown in dotted lines in FIG. 10.

In the apparatus according to FIGURE 11 there is the same polarizer Pand the same quarter-wave plate Q traversed by the incident light ray 14and the reflected light ray 15, a similar compensator C being eventuallytraversed by the same incident light ray and the said reflected lightray which impinges on the observer 0. This apparatus is less versatilethan the apparatus of FIG. 2, being usable for certain isochromaticpatterns but not for isoclinic patterns.

In all the preceding and equally in all that follows, it is to beunderstood that the term observer includes the eye or eyes of one ormore observers, or a telescope, or a microscope through whichobservation can be made, or a so-called zoom" lens, or a combination ofthe three, or any other system such as a photoelectric cell followed ornot by an amplifier which can be attached to a registering device, or anoscillograph or a photographic apparatus, cinematographic apparatus or atelevision camera, etc.

FIGURES 12 and 13 show schematically the arrangement of the polarizer Pand the quarter-wave plate Q as well as their optical axes as related toFIG. 11. FIG. 13, wherein the Q-P sandwich of FIG. 11 is viewed from theposition of the patients eye rather than the opposite viewpoint of FIG.12, shows the axis of the quarter-wave plate to be at 45 to the verticalaxis of the polarizer P (which in this case serves also as an inflexibleanalyzer).

FIGURE 14 relates to another form of apparatus in which the luminoussource S, which can be any of those referred to above, transmits a lightray 16 through a polarizer P and a quarter-wave plate Q which eventuallyfalls on a half-mirror 6;. Then the light rays 17 fall normally orquasi-normally on the eye cornea e, traverses the cornea, then isreflected and/or diffused from their-is, traverses again the cornea andis directed toward the halfmirror G another half-mirror G of the samematerial and thickness as mirror G but not necessarily provided witha'semi-reflective surface, and disposed symmetrical y with respect tothe latter in a plane perpendicular to the direction of the light ray18, eventually a compensator C, a quarter-wave plate Q, then an analyzerA behind which is the observer 0, and which can be any of the types ofobserving or recording apparatus mentioned above. Element G beingperpendicular to element G and disposed at 45 to ray 17, compensates forthe shift of the ray due to diffraction of the ray travelling fromcornea of eye e toward the observer as it enters and then departs fromdiagonal element G In the embodiment according to FIGURE 15 the patientcan himself ascertain the birefringence which his eye 9 shows due to theinterposition of the half-mirror G and the mirror M, the observationbeing able to be equally followed by the observer 0.

In the embodiment of FIGURE 16, a single quarterwave plate Q is providedand it is traversed by the incident light ray 17 as well as by thereflected light ray 18. In this embodiment as well as the precedingembodiments, particularly those of FIGURES 14 through 16, the polarizerP can be omitted and the half-mirror G oriented at 54 (approximately) tothe vertical, thus becoming a polarizer, provided that the source S andpolarizer P are shifted to the right to retain the fundamental equalityof the angle of incidence (of light from source S) and angle ofreflection (of light proceeding along the path to the cornea of eye e).Thus, Brewsters angle provides the polarization.

In the installation of FIGURE 17, one draws from the fact that the angleof incidence on the eye, instead of being normal in an absolute manner,can be substantially normal and the light rays 16 are reflected by amirror M toward the eye e at an angle of incidence such that thereflected or diffused light ray 20 misses the mirror M and reaches theobserver after passing through a quarterwave plate Q and an analyzer Aand eventually a compensator C.

The function of the quarter-wave plate Q, the axis of which is at 45 tothe polarizer axis, is to suppress the isoclinic pattern and hencerender more clearly visible the isochromatic pattern.

In all the preceding installations or apparatus arrangements there canbe envisioned, complementarily, a lens placed in front of the eye toobtain an angle of incidence as normal as possible in one part or in thetotality of the observed surface. Such a lens k (FIGURE 18) can be alsoin contact with the eye through the intermediation of a liquid which canbe lacrymal liquid (i.e., a contact lens can be used).

FIGURE 19 shows an apparatus arrangement for the observer with anoblique angle of incidence. The mounting is similar to that which isshown in FIGURE 2, that is to say, the incident light ray from lightsource S traverses a polarizer P and eventually a quarter-wave plate Qand falls on the eye along a ray which is at a smaller angle to the iris(the surface used for reflecting the ray back through the cornea) thanthe angle in FIG. 2.

FIGURE 20 relates to a variant of an apparatus for observation underoblique incidence, the said apparatus being identical to that which, inFIGURE 2, is shown disposed for the observation at a normal orquasi-normal angle of incidence but which, in FIGURE 20, is arranged forobservation at an oblique angle of incidence.

FIGURE 21 is a variant of the apparatus of FIGURE 20 in which a mirror Mdiverts the reflected or diffused light ray 23 of the incident light ray21.

With an installation arranged for the observation at a normal orquasi-normal angle of incidence, one obtains equally the resultsrelative to an oblique angle of incidence when the patient instead oflooking in the direction of the incident and reflected light rays looksupwardly, down- 7 wardly or to the left or to the right, the instrumentcontinuing to be aimed toward the center of the eyeball.

FIGURE 22 shows, for example, in broken lines an isochromatic line 50observed with an apparatus such as that shown schematically in FIGURE 2,FIGURE 4, FIGURE 11 and FIGURES 14 through 17, but when the patientlooks upwardly this line being an isochromatic line with an obliqueangle of incidence in a vertical plane; the isochromatic line for normalincidence of light is shown at 51. There is shown at 52 in the samefigure the isochromatic line observed when the patient looks toward theleft, this line being an isochromatic line at an oblique angle ofincidence in a horizontal plane. In this figure the line in the middleof the face or nose is designated by n.

For observation or measurements at a predetermined obliqueness one usesan apparatus shown for the observation at a normal angle of incidencewhen the patient looks in the direction of the incident and reflectedlight ray and the patient directs his line of sight toward apredetermined point understood to be distinct from the sensibledirection common to the incident light ray and the reflected light ray.

FIGURE 23 illustrates an installation for the simulta neous observationof birefringence with normal and oblique angles of incidence. The lightray 24 resulting from the reflection or dilfusion of the light ray 25furnishes the observation where the measurement follows the normal orquasi-normal angle of incidence, while the light ray 26 furnishes theobservation where the measurement is at an oblique angle of incidence,the obliqueness of the light ray 27 for reaching observer beingfurnished by mirror M and mirror M which send the light rays 26 backthrough the compensator C and the analyzer A eventually preceded by aquarter-wave plate Q.

In addition to the above, one could include a regulatable diaphragm 61arranged to be turned downward to interrupt beam 26 or upward tointerrupt beam 24, or turned parallel to beam 26 to permit observationby both beams simultaneously.

The installation of FIGURE 24 also permits the observation and themeasurement at an oblique angle of incidence, the incident light ray 29and the light ray 30 being in prolongation of one another. In FIGS. 24,and 26 there is no reflection at the surface of the iris.

In the variant of FIGURE 25 mirror M permits observation from in frontof the patient. In the variant of FIGURE 26 wherein the source ofobservation is also in front of the patient this arrangement ispermitted by virtue of mirrors M and M In all the forms of apparatus forthe observation or measurement at an oblique angle of incidence, theangle of incidence can be either fixed or variable.

Referring now to FIGURE 27 with regard to an em bodiment similar to thatshown in FIGURE 2 but comprising a half-mirror G behind which directobservation can be carried out as shown at 0, the said half-mirrorfurnishes also reflected light rays 31 which fall on a photographic orcinematographic apparatus 32.

In the embodiment of FIGURE 28 similar to that of FIGURE 27 the lightrays 31 fall on a photoelectric cell PH which controls a measuring orobservation device 33 such as an oscillograph or oscilloscope. In thisembodiment in addition the electric current furnished by thephotoelectric cell PH is utilized to control a slave system 34 acting oncompensator C to maintain a null balance and thus to furnish directlyquantitative information.

In the modification of FIGURE 29 the light rays 31 fall on a televisioncamera 34', a television receiver being shown schematically at 35.

In all embodiments the compensator C can be remotely actuated ifnecessary.

In all forms of apparatus monochromatic or partially chromatic filterscan be mounted in front of light source S or observer 0.

In the installation of FIGURE 30 a plurality of observers O O 0 arelocated behind the polarizers P P P eventually preceded each by aquarter-wave plate Q, the incident light ray being derived from a commonluminous source S positioned in front of a polarizer P eventuallyfollowed by a quarter-wave plate Q.

In the installation of FIGURE 31, observer O and observer 0 see thelight furnished by light source S, after diffusion by the iris of theeye e, observer 0; receives the light rays emanated from source S; andafter diffusion of the light rays by the iris of the eye the observer O,receives the light rays emanating from light source S after diffusion bythe iris of the eye. In all forms of apparatus the polarizer, theanalyzer, the quarter-wave plates and the compensator can turn in asuitable plane independently of one another or coupled. The apparatuscan also be devoid of quarter-wave plates and compensator.

The invention envisions also the determination of the opticalcoefiicient of stress and/or strain by application of one or the otherof the following formulas:

1-2=5/tk o' o' =5/tC In these formulas k is the strain optical constant;C is the stress optical constant; e -6 is the difference between thesecondary principal strains; o' u is the difference between thesecondary principal stresses; 6 is the relative retardation of thenormal or oblique angle of incidence; and t is the length of the opticalpath in the birefringent medium.

FIGURE 32 shows an arrangement for determining constants such as C andk. The eye e is surrounded at a distance by a transparent wall 36disposed in a fluid-tight manner along arcuate opening 37. There can bemade present in the interior of chamber 38 a known pressure by a pump 39and the observation of birefringence in place, for example, of normal orquasi-normal angle of incidence as indicated. The pressure in theinterior of chamber 38 being known, for example, by a manometer 40 oronly the pressure increase, the observations or measurements made byobserver 0 in the direction indicated above permitting determination ofthe constants. The changes of the isochromatic patterns may becorrelated with dilferent applied pressures as measured by manometer 40between wall 36 and the cornea. In this manner, for example, one maydetermine how much change of exterior pressure on the cornea is requiredto produce a change of 10% in its strain (and the stress producing same)as observed. C and k can also be obtained by applying a known force to agiven portion of the eye.

FIGURE 33 shows the main chracteristics of the fringe pattern of anormal eye observed in accordance with the present invention and wherethere are 2 singular points (No. 0) on either side of pupil 3' and afringe line (No. 1) seen by the observer during the examination of anormal eye with a crossed circular polariscope in quasinormal incidence.In subsequent FIGURES 34 through 37, observations are made under thesame conditions as in FIGURE 33 but for abnormal eyes.

In FIGURE 34 the fringe patterns of the eyes being observed clearly showabnormalities which are due to glaucoma or other eye disease. The fringepattern of the eye left shows that fringe No. 1 is much closer to theperiphery 5 than is the case for the normal eye of FIG- URE 33 and, inaddition, the fringe line is somewhat distorted. The fringe pattern theright eye shows at least three fringe lines Nos. 1, 2 and 3, thus alsoindicating an abnormal eye condition.

In FIGURE 35 the fringe pattern of the left eye has 3 /2 fringes andfringe No. 1 has an irregular zig-zag outline. The right eye has noorganized fringe pattern at all.

In FIGURE 36 the fringe pattern of the left eye has an abnormal fringepattern in portions only and in the other portions there is no organizedbirefringence, whereas in the right eye one portion thereof issubstantially normal and the other portion shows increased birefringenceby the presence of fringe lines Nos. 1, 2 and 2.7.

In FIGURE 37 the fringe pattern of the left eye is normal but the fringepattern of the right eye shows no organized birefringence in one portionof the eye and an abnormal pattern of fringe lines Nos. 1 and 2 inanother portion of that eye. Thus showing that birefringence in the twoeyes of one and the same person can have completely differentdistributions and values.

A linear, circular or curvilinear scale similar to the fringe contour ofa normal eye is provided to measure the location of each of the observedfringes and its displacement or variation from normal form and positionas explained below anent FIGURES 38 and 39.

FIGURE 38 represents a typical scale wherein a screen or photograph orother base 53 has marked thereon a scale 54 which as will be noted fromFIGURE 33 represents the fringe pattern of a normal eye and,consequently, the fringe portion 55 of the examined eye can besuperimposed and its variance from 54 determines the extent ofabnormality, i.e., the displacement and/ or distortion of the fringeportion of the examined eye.

In FIGURE 39 a composite scale is represented on the same screen,photograph or other base 53 but whereon there is a circular graduatedscale 56, a linear graduated scale 57 and 'a radial scale 58 so thatwhen the fringe pattern 59 of the examined eye is superimposed precisemeasurements can be made and the abnormalities are immediately apparent.

The fringe patterns of FIGURES 34 through 37 are those actually observedin accordance with the present invention in patients who had beendiagnosed as having glaucoma in one or both eyes. It will be clear,however, that the present invention makes it possible to observe,determine and measure eye abnormalities whether they result fromglaucoma or other eye disease or even from some systemic cause and afterhaving examined a number of eyes both normal and abnormal, the observeris in an excellent position to make a tentative diagnosis or to referthe patient to an ophthalmologist. The fringe patterns can be visualizedand recorded as by projecting them on a screen or taking photographs orin any other desired manner so as to have a record. It has notheretofore been possible to accomplish these objectives in a simple,rapid and effective manner and while polariscopes of various designs areper se known, they have never been applied to the observation of eyes orother human or animal tissue for the present purposes. It is also to beunderstood that the present invention makes it possible to observe botheyes of a patient simultaneously and to make a comparison as well asexamining one eye at a time. It is further to be understood that thepolariscope arrangements described above and illustrated in the drawingsare intended as typical examples and not as an exhaustive compilation ofall possible polariscope arrangements.

What is claimed is:

1. A method for investigating the physiological and health condition ofthe living eye through photoelastic patterns which comprises projectinpolarized light through the cornea to the iris to produce reflection oflight therefrom, the cornea producing birefringence of the light passingtherethrough, and receiving and analyzing in a polarizer the lightreflected back through the cornea by the iris, whereby the stresses andstrains in the cornea produce birefringence resulting in significantpatterns of the received and analyzed light.

2. A method for determining the strain pattern in the frontal portion ofthe human eye through photoelastic fringe patterns which comprisesdirecting a beam of light through a polarizer into the cornea of the eyeat a me selected angle to cause a portion of the light to reach the irisand be reflected back thereby, the cornea producing birefringence of thelight passing therethrough, receiving light emerging from the cornea ofthe eye as a result of reflection by the iris, and passing the receivedlight through a compensator and an analyzer to present an image whereinthe forms and relative positions of the fringes are related to thedistribution of strain in the cornea.

3. A method for testing the living eye with respect to its physiologicalcondition which comprises directing a beam of light from a light sourcethrough a polarizer and a contiguous quarter-wave plate and thencethrough the cornea of the eye so as to cause the beam of light to maketwo traversals through the cornea, one as it enters and the other as itexits from the eye, whereby the strain in the cornea producesbirefringence of the light as it enters the cornea and emergestherefrom, said light then being caused to pass successively through acompensator, a quarter-wave plate and an analyzer to render visible toan observer the patterns of birefringence related to the condition ofthe eye.

4. A method in accordance with claim 3, in which the observedbi-refringent pattern is recorded and the number and relative locationsof the thus revealed fringes are measured for diagnostic purposes.

5. A method in accordance with claim 4, in which the bi-refringentpattern is made photographically permanent.

6. A method according to claim 3, in which an external pressure isapplied to the eye, whereby the photoelastic results and tonornetrlceffects may be correlated.

7. A method in accordance with claim 1, in which a plurality of beams oflight are directed upon the cornea of the eye at preselected differentincident angles and the separate reflected and diffused light beams areobserved for interpretation.

8. A method in accordance with claim 1, in which an external pressure isapplied to the eye, whereby the photoelastic results and tonometriceffects may be correlated.

9. A method for investigating the physiological and health condition ofthe living eye through photoelastic patterns which comprises projectingpolarized light through the cornea to the iris to produce reflection oflight therefrom, the cornea producing birefringence of the light passingtherethrough, and receiving and analyzing the light reflected backthrough the cornea by the iris whereby the stresses and strains in thecornea produce birefringence resulting in significant patterns of thereceived and analyzed light.

References Cited UNITED STATES PATENTS 2,735,423 2/ 1956 Powell 128-7653,070,087 12/ 1962 Sittell 128-2 2,475,706 7/1949 Jamiesen 33-1743,061,936 11/1962 Dobbeleer 33-174 2,837,086 6/1958 Thorburn 128-7653,096,767 7/1963 Gresser et al 128-395 2,625,850 1/1953 Stanton 88-143,062,087 11/1962 Zandman et al. 88-14 3,096,388 7/ 1963 Davenport 88-14OTHER REFERENCES Ultrasonics in Ocular Diagnosis, G. H. Mundt, AmericanJournal of Ophthalmology, March 1956, pp. 488-493.

RICHARD A. GAUDET, Primary Examiner.

S. BRODER, Assistant Examiner.

9. A METHOD FOR INVESTIGATING THE PHYSIOLOGICAL AND HEALTH CONDITION OFTHE LIVING EYE THROUGH PHOTOELASTIC PATTERNS WHICH COMPRISES PROJECTINGPOLARIZED LIGHT THROUGH THE CORNEA TO THE IRIS TO PRODUCE REFLECTION OFLIGHT THEREFROM, THE CORNEA PRODUCING BIREFRINGENCE OF THE LIGHT PASSINGTHERETHROUGH, AND RECEIVING AND ANALYZING THE LIGHT REFLECTED BACKTHROUGH THE CORNEA BY THE IRIS WHEREBY THE STRESSES AND STRAINS IN THECORNEA PRODUCE BIREFRINGENCE RESULTING IN SIGNIFICANT PATTERNS OF THERECEIVED AND ANALYZED LIGHT.